Cyclic hardening of an Ni-14.4 at. pct AI alloy containing coherent precipitates

Abstract

The cyclic stress-strain response and microstructures (TEM) were studied in specimens of Ni-14.4 at. pct Al alloy heat treated to obtain a range of coherent precipitate sizes from underaged (very small precipitates, 6 nm) to overaged (large precipitates, 46 nm). Overaged specimens cyclically hardened initially to a plateau or stable stress amplitude until crack initiation while underaged samples, after initial cyclic hardening, softened prior to crack initiation. There was very slight cyclic softening in peak-aged samples prior to failure. An inverse exponential equation modeled the cyclic hardening behavior of all specimens tested. An empirical constant, R, which measured the rate of approach to the cyclic hardening maximum, changed significantly with precipitate size, being maximum for the peak-aged condition. The total cyclic hardening was also maximum for peak aging. The microstructural examination showed that the plastic deformation during early cyclic straining was most uniform in the peak-aged samples compared to under- or overaged samples. This correlates with the faster approach to maximum hardening. Furthermore, initial paired dislocation motion was replaced by unpaired tangles as cycling progressed.